Fixie culture in the bigger context... a discussion.

The general concensus in the US seems to be that fixies lost approximately 80%of their credibility factor the day that Walmart started selling them.

Fixies clearly had no credibility if the bicycles that Walmart sell define or refine their credibility. The conjecture is, I'd suggest, more about American culture than about bicycles.

Now they are dying out at an astonishing rate, with fixie riders being seen as rather juvenile and silly,

Nothing new. if you rode a bicycle beyond the age of 16 you were viewed as a bit nutty. Sure there were a few bike booms.. but most of those booms never went for the mainstream beyond their short lives.. Cyclists were always a niche sub-culture subject often to aggression from motorists.

among the new wave of riders on their Linus, Public and Civias, sneering over their leather-clad porteur bars grips and cream Schwalbes.

Bicycles for non-cyclists.

The 1960s Mod culture is a good example, morphing as it did in short order, from Italian suits and Modern Jazz, to Parkas and fighting.

There was never a 1960s mod culture. Its a perception of a culture that once existed that defined a "Mod revival". And the roots? East-End jewish kids in the 1950s.. The 1960s mod was nothing more than a working class marketing strategy defined through mass media. Even the 1950s East-London clinque had its roots elsewhere.. namely in Germany. In the 1930s Jazz and English crepe soled shoes were in Hamburg and Munich--- what has come to be called "fashion statements"-- political expressions counter to the fashions of the Nationalists, Royalists, Communists and Fascists.

Smoking and growing weed was once the preserve of the hippie intelligencia

New to me that poor ghetto blacks in the 1930s where "hippie intelligencia". Reefer was a popular drug among the poor.

The concept of "Designer" (particularly in the form of mass market clothing rather than the work of the original creator) really began as an offshoot from those Mods, being reborn in the Yuppie era, before descending into its modern place in the mail-order catalogues and shopping malls of working-class suburbia.

Hardly. "Designer" clothing existed long before the great great grandparents of any Yuppies were born. Fashion in Europe, however, used to be regulated. People were, for example, forbidden to wear clothing outside of the class. Velvet, short jackets, certain furs etc. were all reserved. The basis of modern youth culture I'd put not in the 20th but in the 18th century--- on the heels of emancipation. The Dandys were quite a sensation and Bo Brummell perhaps one of the first mass pop stars. Johann Wolfgang von Goethe, for example, used to try to copy Brummell's style.